1. Field of the Invention
The present invention relates to a portable terminal capable of switching a display memory such as VRAM based on data displayed on a screen.
2. Related Art
Along with recent tendency toward a high-definition display device (display) in a portable terminal such as a cellular phone, an allowable maximum amount of data displayed on a screen has been increased and much information has been provided to a user at a time. However, the increase in amount of data displayed on the screen leads to a longer update time. As a result, responsiveness to a user operation is deteriorated.
In a case where a refresh rate for refreshing a display and a display memory (hereinafter referred to as “frame buffer”) are paired (i.e., single buffer case) in a portable terminal, processing for writing data to the frame buffer (update) and processing for reading display data are executed in an asynchronous fashion. In this case, if the update direction for writing and the update direction for reading are the same, and the writing processing and the reading processing are executed at the same time, or if the update direction for writing and the update direction for reading are at 90 degrees, tearing occurs upon each updating.
The term “tearing” means such a phenomenon that since a timing at which screen data is updated and a timing at which display data is read are not synchronized, screens generated at different timings are concurrently displayed in upper and lower portions or on the right and left portions of the display, and a user catches flickers on the display. In general, if the writing direction and the reading direction cross each other at 0 degrees, tearing appears linearly in the same direction as the updating direction. On the other hand, if the writing direction and the reading direction cross each other at 90 degrees, tearing appears diagonally.
To avoid such tearing, Japanese Unexamined Patent Application Publication No. 2006-171488 discloses an image display device that uses two frame buffer areas for displaying an image according to a double-buffer method under a normal condition or just before image transfer to a nonvolatile memory, and at the time of transferring an image to the nonvolatile memory, switches the double-buffer method to a single-buffer method to use one of the frame buffer areas and transfers an image from the other frame buffer not used for image display toward a nonvolatile memory NVRAM, for example.
In general, at the time of displaying a screen on a display of a portable terminal, a frame buffer IC (driver) for controlling image data (for example, VRAM) to be finally outputted to the display uses two or more VRAMs to separately prepare a VRAM for current display and a VRAM for updating to thereby control display. If the portable terminal includes two or more VRAMs, the entire screen area should be updated each time to synchronize displayed data. On the other hand, in the case of using one VRAM, it is only necessary to update an area corresponding to a difference from a previous screen. As a result, a processing time can be shortened, but the updating processing and display processing are performed using the same VRAM, thus leading to tearing and reduction in image quality of a display screen.
Further, the reduction in image quality of a display screen is ignorable depending on displayed data. Thus, if a VRAM control method can be switched according to displayed data, a performance for the display processing could be increased. In this case, it is desirable to select a VRAM control method without modifying a higher-level application program.